A serial-type image forming apparatus is a type of image forming apparatus such as a printer, a facsimile, a copier, a plotter, or a multifunction copier having functions of a printer, facsimile, and copier. A serial-type image forming apparatus normally includes one or more recording heads (print heads) mounted on a carriage which recording heads are made of liquid drop spraying heads that spray drops of recording liquids (for example, inks). In such a serial-type image forming apparatus, the carriage is moved to serially scan a recording medium (hereafter called a paper sheet, but not limited to a sheet of paper, and may also be called recording paper, a transfer medium, a printing medium, or the like) in a direction orthogonal to the direction in which the recording medium is conveyed; and the recording medium is conveyed intermittently a recording width at a time. An image is formed (recorded or printed) on the recording medium by repeating conveying and recording steps alternately.
In such an image forming apparatus, if the edges of a paper sheet are detected incorrectly, liquid drops may be sprayed onto an area outside of the paper sheet. This may deteriorate the image quality or smear a conveying unit such as a conveyor belt. Patent document 1 discloses an image forming apparatus that has a paper sheet sensor mounted on a carriage for scanning a paper sheet widthwise and is thereby able to accurately detect the edges of the paper sheet on a plane close to the image forming area.    [Patent document 1] Japanese Patent Application Publication No. 2004-237693
In an image forming apparatus having recording heads for spraying liquid drops, the nozzles of the recording heads and a paper sheet sensor for detecting the leading edge of a paper sheet must be aligned appropriately. However, because of irregularities in assembling, the distance between the recording head nozzles and the paper sheet sensor may vary. This makes it necessary to align the recording head nozzles and the paper sheet sensor (this process is called “registration adjustment”).
On the other hand, the demand is high for an increased printing speed of image forming apparatuses. When printing multiple pages, the printing speed can be increased by narrowing the distance between a preceding paper sheet and a succeeding paper sheet (hereafter called a distance between paper sheets). However, in a configuration where a sensor for detecting the leading edge of a paper sheet is mounted on a carriage, there is a time gap between when the scanning of a preceding paper sheet is finished and when the detection of the leading edge of a succeeding paper sheet is started, because the carriage has to be moved to a specified position so that the sensor is able to detect the leading edge of the succeeding paper sheet. This time gap makes it difficult to reduce the distance between paper sheets below a certain level. In other words, improvement in techniques for detecting the leading edge of a paper sheet has a great effect in improving the printing speed of an image forming apparatus.
One way to solve the above mentioned problem is to provide a second paper sheet sensor placed upstream from a first paper sheet sensor on the carriage in the paper conveying direction and thereby to detect the leading edge of a paper sheet using the first and second paper sheet sensors.
However, providing multiple paper sheet sensors for detecting the leading edge of a paper sheet makes it necessary to perform registration adjustment for each of the paper sheet sensors and therefore complicates the process of registration adjustment.